The horizon problem refers to the former mystery of why the Universe is homogeneous and isotropic despite distant regions of space failing to be causally connected. Science has since provided an explanation.

Scientific circles know the problem with the Big Bang that the creationists refer to as the horizon problem. The horizon problem states that the universe is homogeneous and highly isotropic (it's largely the same in every direction we look) despite insufficient time for regions far away from each other to "communicate" and become roughly the same in temperature, density, and other properties. However, an explanation consistent with these and other observations has existed since the early 1980s.

The explanation, known as inflation, was proposed by Alan Guth and holds that the Universe underwent a brief period of rapid accelerated expansion very early in its history. Inflation accounts for the observed homogeneity and isotropy of the Universe, as well as the flat geometry of the Universe. Furthermore, inflation predicts very small but observable differences in temperature of the cosmic microwave backgroundradiation, which have since been confirmed by satellite-based measurements.[2]

The inflationary Big Bang theory continues to be, overwhelmingly, the best scientific explanation for the beginning of the observable universe.